An isolated DC/DC converter, specifically, a flyback type or forward type DC/DC converter, is used in various power source circuits including an AC/DC converter. FIG. 1 is a circuit diagram of an AC/DC converter 100r including a synchronous rectification type flyback converter 200r. 
The AC/DC converter 100r mainly includes a fuse 102, an input capacitor Ci, a filter 104, a diode rectifier circuit 106, a smoothing capacitor Cs, and the flyback converter 200r. 
A commercial AC voltage VAC is input to the filter 104 via the fuse 102 and the input capacitor Ci. The filter 104 removes the noise of the commercial AC voltage VAC. The diode rectifier circuit 106 is a diode bridge circuit that full-wave rectifies the commercial AC voltage VAC. An output voltage from the diode rectifier circuit 106 is smoothed by the smoothing capacitor Cs and converted into a DC voltage VIN.
The isolated flyback converter 200r receives the DC voltage VIN by an input terminal P1, steps down the same, and supplies an output voltage VOUT stabilized to a target value to a load (not shown) connected to an output terminal P2.
A switching transistor M1 is connected to a primary winding W1 of a transformer T1. A rectifier diode D1 is connected to a secondary winding W2 of the transformer T1, and an output capacitor Co1 is connected to the output terminal P2. A feedback circuit 206 drives a light emitting element of a photocoupler 204 by a current corresponding to a difference between the output voltage VOUT and its target voltage VOUT(REF). A feedback current IFB corresponding to the difference flows through a light receiving element of the photocoupler 204.
A rectifier diode D2 and a smoothing capacitor Co2 form a power source circuit 208 together with an auxiliary winding W3 of the transformer T1. A source voltage VCC generated by the power source circuit 208 is supplied to a power (VCC) terminal of a primary side controller 300r. 
The primary side controller 300r is a pulse width modulation (PWM) controller. A feedback signal VFB corresponding to the feedback current IFB is generated at a feedback (FB) terminal of the primary side controller 300r. Further, a current detection signal VCS proportional to a primary current IP flowing through the switching transistor M1 is feedback to a current detection (CS) terminal of the primary side controller 300r. The current detection signal VCS is a voltage drop of a sense resistor RCS installed in series with the switching transistor M1.
For example, the primary side controller 300r includes a pulse width modulator of a peak current mode, and generates a pulse signal SOUT having a constant frequency (period) and having a duty ratio corresponding to the feedback signal VFB and the current detection signal VCS to drive the switching transistor M1 connected to an output (OUT) terminal.
The present inventor has reviewed an operation of the DC/DC converter 200r of FIG. 1 in a state where the output voltage VOUT is low, and recognized the following issues. The output voltage VOUT is low, for example, immediately after the startup or when the output of the DC/DC converter 200r is grounded.
FIG. 2 is a waveform diagram after the start of the DC/DC converter 200r of FIG. 1. The switching transistor M1 is turned on in synchronization with an oscillator clock (a set signal SSET) of a constant period TOSC. When the switching transistor M1 is turned on, the primary current IP flows through the primary winding W1. The primary current IP increases with a slope VIN/LP proportional to an input voltage VIN. LP is the inductance of the primary winding W1.
When the primary current IP reaches a peak level corresponding to the feedback signal VFB, the switching transistor M1 is turned off. During an OFF period of the switching transistor M1, a secondary current IS flows through the secondary winding W2. The secondary current IS decreases with a slope (VOUT+VF)/LS. LS denotes the inductance of the secondary winding W2 and VF denotes a forward voltage of the rectifier diode D1.
Thereafter, when the set signal SSET is asserted, the switching transistor M1 is turned on again. The primary current IP starts to rise with a current amount of the secondary current IS immediately before the switching transistor M1 is turned on, as a starting point. The DC/DC converter 200r repeats this operation.
A decrement ΔIS of the secondary current IS in an OFF time TOFF is ΔIS=(VOUT+VF)/LS×TOFF. Since the output voltage VOUT is low immediately after the startup, the slope of the secondary current IS in the OFF time TOFF is small, and therefore, the decrement ΔIS is small. Thus, the secondary current IS does not drop to zero during the OFF time TOFF.
When this operation repeats, the DC/DC converter 200r starts in a continuous mode and large currents IP and IS flow through the primary winding W1 and the secondary winding W2. When the DC/DC converter 200r starts in the continuous mode, a very high surge voltage exceeding 100V is generated across the rectifier diode D1 of the secondary side. In addition, a drain voltage VDS1 of the switching transistor M1 increases. Further, the same phenomenon occurs not only at the startup but also when the output is grounded.
Therefore, in order to cope with the operation in the current continuation mode in a low output state, conventionally, it was necessary to devise the transformer T1 or to design a withstand voltage and the like of the switching transistor M1 or the rectifier diode D1 to be high, leading to an increase in cost.